The present invention relates to systems for locating vehicles travelling within a prescribed area or over a prescribed route, and particularly to such systems wherein road side sensors receive emitted signals from vehicles and transmit the signals to a central decoding station. The invention as described herein is an improvement over the system described in my prior U.S. Pat. No. 3,568,161 which is incorporated herein by reference.
The system described in my prior patent employs a coded emitter located in each vehicle and provides street-side sensors installed at pre-selected locations within an area or region being monitored. The emitter is a very low power RF transmitter which continuously radiates a signal modulated by audio coding tones which identify the vehicle and/or its status. The signal is demodulated at the sensor and automatically transmitted to a terminal center by telephone lines or the like. Processing at the center permits display or other type readout of the location of each vehicle since the particular vehicle code has been received at a particular sensor location. Vehicle location is updated each time the vehicle passes a sensor. The number of vehicles which can be unambiguously identified in such a system depends on the number of coding tones utilized in each identification code. It is of course possible to increase the vehicle capacity of the system by using a sequence of coding intervals wherein different combinations of coding oscillators are gated on or not during each coding interval. The problem with this approach, however, is that failure of a coding oscillator can provide an erroneous identification signal, resulting in the anomaly of the same vehicle showing up at the two locations within the monitored region. The anomaly may be avoided by using a parity oscillator which is gated on or not during each coding interval to assure that an even (in the case of even parity) or odd (in the case of odd parity) number of coding tones are gated on at any time. However, it is desirable to avoid the expense of an additional parity oscillator. In fact, it is desirable to minimize the number of oscillators required overall so that the cost of the system can be minimized.
It is therefore an object of the present invention to provide a coding arrangement in the system of the type described wherein the number of coding oscillators, and therefore the system expense, is kept to a minimum.
It is another object of the present invention to provide a coding sequence in a vehicle locator system of the type described wherein the number of oscillators is kept to a minimum and wherein the sequence is repeated sufficiently often to assure that a complete coding sequence is received by each sensor station passed by the vehicle.
It is another object of the present invention to provide a party line arrangement in the connections between the sensor stations and the central office to thereby minimize the cost of the system.
It is another object of the present invention to adapt the system of the type described to large geographic regions by utilizing long distance telephone interconnections between the sensor stations and the central processing office.
It is another object of the present invention to employ radio links between call boxes and decoding stations in a vehicle locator system.
It is still another object of the present invention to provide an arrangement in a vehicle locator system of the type described wherein transmission of a vehicle coding signal is delayed when two vehicles are in close proximity so that confusion is minimized at the decoder.